Customize: Ensure that `WP_Customize_Manager::save_changeset_post()` returns...

Customize: Ensure that `WP_Customize_Manager::save_changeset_post()` returns `setting_validities` even for supplied values that are unchanged from values in changeset.

Check setting existence and authorization via `WP_Customize_Manager::validate_setting_values()` even for `null` values to account for custom params being added to settings, preventing failures from being silently ignored.

See #38705, #30937.
Fixes #38865.

Built from https://develop.svn.wordpress.org/trunk@39320


git-svn-id: https://core.svn.wordpress.org/trunk@39260 1a063a9b-81f0-0310-95a4-ce76da25c4cd